Image resolution

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Image resolution describes the detail an image holds. The term applies equally to digital images, film images, and other types of images. Higher resolution means more image detail.

Image resolution can be measured in various ways. Basically, resolution quantifies how close lines can be to each other and still be visibly resolved. Resolution units can be tied to physical sizes (e.g. lines per mm, lines per inch) or to the overall size of a picture (lines per picture height, also known simply as lines, or TV lines). Furthermore, line pairs are often used instead of lines. A line pair is a pair of adjacent dark and light lines, while lines counts both dark lines and light lines. A resolution of 10 lines per mm means 5 dark lines alternating with 5 light lines, or 5 line pairs per mm. Photographic lens and film resolution are most often quoted in line pairs per mm.

Contents 1 Resolution of digital images 1.1 Pixel resolution 1.2 Spatial resolution 1.3 Spectral resolution 1.4 Temporal resolution 1.5 Radiometric resolution 2 Resolution in various media 3 External links 4 See also

Resolution of digital images

The resolution of digital images can be described in many different ways.

Pixel resolution

The term resolution is often used as a pixel count in digital imaging, even though American, Japanese, and international standards specify that it should not be so used, at least in the digital camera field. An image of N pixels high by M pixels wide can have any resolution less than N lines per picture height, or N TV lines. But when the pixel counts are referred to as resolution, the convention is to describe the pixel resolution with the set of two positive integer numbers, where the first number is the number of pixel columns (width) and the second is the number of pixel rows (height), for example as 640 by 480. Another popular convention is to cite resolution as the total number of pixels in the image, typically given as number of megapixels, which can be calculated by multiplying pixel columns by pixel rows and dividing by one million. Other conventions include describing pixels per length unit or pixels per area unit, such as pixels per inch or per square inch. None of these pixel resolutions are true resolutions, but they are widely referred to as such; they serve as upper bounds on image resolution.

Below is an illustration of how the same image might appear at different pixel resolutions, if the pixels were poorly rendered as sharp squares (normally, a smooth image reconstruction from pixels would be preferred, but for illustration of pixels, the sharp squares make the point better).

Image:Resolution illustration.png

Spatial resolution

The measure of how closely lines can be resolved in an image is called spatial resolution, and it depends on properties of the system creating the image, not just the pixel resolution in pixels per inch (ppi). For practical purposes the clarity of the image is decided by its spatial resolution, not the number of pixels in an image.

The spatial resolution of computer monitors is generally 72 to 100 lines per inch, corresponding to pixel resolutions of 72 to 100 ppi.

In Geospatial Information Systems (GIS), Spatial Resolution commonly refers to the Ground Sample Distance (GSD) of an image. Or in other words, how much of the earth's surface a single pixel covers.

Spectral resolution

Color images distinguish light of different spectrum. Multi-spectral images resolve even finer differences of spectrum or wavelength than is needed to reproduce color. That is, they can have higher spectral resolution.

Temporal resolution

Movie cameras and high-speed cameras can resolve events at different points in time. The time resolution used for movies is usually 15 to 30 frames per second (fps), while high-speed cameras may resolve 100 to 1000 fps, or even more.

Radiometric resolution

Radiometric resolution determines how finely a system can represent or distinguish differences of intensity, and is usually expressed as a number of levels or a number of bits, for example 8 bits or 256 levels which is typical of computer image files. The higher the radiometric resolution, the better subtle differences of intensity or reflectivity can be represented, at least in theory.

Resolution in various media

• DVDs are 720 by 480 (NTSC) pixels or 720 by 576 (PAL) pixels
• High definition television is 1920 by 1080 pixels or 1280 by 720 pixels
• 35 mm film is scanned for release on DVD at 1080 or 2000 lines as of 2005.
• 35 mm original camera negative motion picture film can resolve up to 6,000 lines.
• 35 mm projection positive motion picture film has about 2,000 lines which results from the analogue printing from the camera negative of an interpositive, and possibly an internegative, then a projection positive.
• Sequences from newer films are scanned at 2,000, 4,000 or even 8,000 columns (line measured the other directions), called 2K, 4K and 8K, for quality visual effects editing on computers.

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